This invention relates to energy beam processing of a workpiece and, more particularly, relates to the application of high energy beams onto silicon wafers in a semiconductor processing system.
In the fabrication of integrated circuits, a number of processes have been established which involve the application of high energy beams onto silicon wafers. These processes include ion implantation, ion beam milling and reactive ion etching. In each instance a beam of ions is generated in a source and directed with varying degrees of acceleration towards a target.
In commercial semiconductor processing one of the major objectives is to achieve a high throughput in terms of wafers processed per unit time. One of the ways to achieve high throughput in an ion beam system is to utilize a relatively high power beam. Associated with the use of a high power beam, however, is the problem of wafer heating. Photoresist, which is commonly present on the wafer surface during processing, has a melting point of about 135.degree. C. Thus, the wafer cannot be permitted to reach 135.degree. C. during processing.
As a consequence, most industrial equipment which is capable of producing high beam power processes wafers in a batch in order to spread the incident power over a large area and to reduce the heating in any given target. A batch processing system involving mechanical movement of the wafers during implantation is disclosed in U.S. Pat. No. 3,778,626 issued Dec. 11, 1973 to Robertson. A batch processing system which utilizes a dual end station to improve throughput is disclosed in U.S. Pat. No. 4,276,477 issued June 30, 1981 to Enge. Batch processing systems are generally large to accommodate the batches and are generally used only for high dose implantations. In addition, throughput is less than optimum because of the time required to manually change batches.
Conductive cooling has also been used to alleviate the problem of wafer heating. For example, wafers are placed in thermal contact with cooled metal platens. Wafers also have been pressed against pliable thermally conductive polymers to enhance thermal contact as disclosed in U.S. Pat. No. 4,282,924, issued Aug. 11, 1981 to Faretra. Another approach has been to introduce a gas behind a wafer in order to permit conduction between the backside of the wafer and the cooled support surface as disclosed in U.S. Pat. No. 4,261,762, issued Apr. 14, 1981 to King. Centrifugal forces have also been used to press wafers against cooled surfaces as disclosed in pending application Ser. No. 284,915, filed July 20, 1981 and assigned to the assignee of the present application. These approaches require elaborate hardware and coolant feedthroughs, do not always provide the desired level of cooling and generally reduce wafer throughput.
It is, therefore, an object of the present invention to provide an ion implantation method and apparatus wherein a beam is time-shared among semiconductor wafers by beam scanning rather than by mechanical movement of the wafers.
It is an additional object of the present invention to provide a method and apparatus for reducing semiconductor wafer heating during high energy beam implantation.
It is yet another object of the present invention to provide an ion implantation method and apparatus which permits a single wafer in one target position to be processed at one time in alternation with wafers in other target positions.